In frequency bands of 2.4 GHz and 5 GHz, wireless LAN systems compliant the IEEE802.11 standard using a frequency band of 20 MHz are becoming widespread at an accelerating pace. The IEEE802.11 standard (IEEE802.11a/b/g) which has already been put to practical use adopts an access scheme of CSMA/CA (carrier sense multiple access with collision avoidance).
Furthermore, the Task Group n (TGn) is engaged in new standardization activities aiming at the achievement of throughput of 100 Mbps or higher and is studying, for example, a MIMO (Multi Input Multi Output) technique that carries out communication using a plurality of antennas and an aggregation technique that aggregates a plurality of packets into one packet to be transmitted/received in a MAC layer.
The activities intended to speed up communication of wireless LAN systems are believed to continue, and in practice, an IEEE802.11 standardization conference as a study group has started to study on the further speed enhancement and improvement of the system capacity of wireless LAN systems.
As one of schemes leading to the improvement of system capacity of a wireless LAN system, there is known a Space Division Multiple Access (SDMA) scheme, that is, a scheme whereby data is transmitted/received between a radio base station and a plurality of radio terminals simultaneously and at the same frequency (spatially multiplexed) (e.g., JP-A 2003-52079 (Kokai)).
When a wireless LAN system is constructed by adopting this SDMA scheme and the CSMA/CA access scheme adopted in the IEEE802.11 standard, there can be a situation in which a radio base station cannot spatially separate packets, which are spatially multiplexed and transmitted, from a plurality of radio terminals and cannot receive packets.
When, for example, the radio base station transmits data packets of different packet lengths spatially multiplexed to the plurality of radio terminals, since response packets are sent back from the respective radio terminals at different timings and this results in interference. If the radio base station is assumed to transmit the next data packet after a lapse of a predetermined time after the reception of the response packets, the radio base station transmits the next data packet and receives the response packets simultaneously, and this also results in interference (collision).
In order to solve these problems, studies are underway on methods for preventing the radio base station from transmitting data packets and receiving response packets simultaneously.
There are disclosed, for example, a method whereby packets are multiplexed and transmitted from a radio base station to a plurality of radio terminals (over a downlink) at the same packet transmission timing and its period (e.g., JP-A 2005-39728 (Kokai)) and a method whereby packets are multiplexed and transmitted from a plurality of radio terminals to a radio base station (over an uplink) at the same packet transmission timing and its period (e.g., JP-A 2003-52079 (Kokai)).
Furthermore, there is also disclosed a method of identifying a group of a plurality of radio terminals capable of multiplexing and transmitting packets to a radio base station under an SDMA scheme (e.g., JP-A 2007-208522 (Kokai)).
The IEEE802.11 standard defines a mechanism whereby when a radio communication apparatus (including a radio base station, radio terminal) receives a packet directed to another radio communication apparatus, a NAV (Network Allocate Vector) is set so as to prevent transmission of packets for a certain period of time.
Therefore, when constructing a wireless LAN system compliant with the IEEE802.11 standard and adopting an SDMA scheme, it is necessary to take into consideration that the radio communication apparatus which has received a packet directed to the other radio communication apparatus prevents transmission of the packet for a certain period of time. However, e.g., JP-A 2003-52079 (Kokai), JP-A 2005-39728 (Kokai) and JP-A 2007-208522 (Kokai) do not take into consideration, when constructing a wireless LAN system adopting an SDMA scheme, that the radio communication apparatus which has received a packet directed to the other radio communication apparatus prevents transmission of the packet for a certain period of time.
Furthermore, the IEEE802.11e standard defines a period (TXOP (Transmission Opportunity) period) during which packets are transmitted/received continuously without relinquishing a transmission access right and allows a plurality of packets to be transmitted continuously within the TXOP period.
That is, radio communication apparatuses compliant with the IEEE802.11e standard can transmit a packet within a TXOP period, receive a response packet to the packet and then transmit packets continuously without acquiring any transmission access right again after a lapse of a predetermined time (e.g., time interval between packets defined by the IEEE802.11 standard MAC protocol specification (SIFS: Short Inter Frame Space)).
However, the wireless LAN system compliant with the IEEE802.11 standard and adopting an SDMA scheme does not consider the method whereby a plurality of radio terminals spatially multiplex and transmit packets to the radio base station and continuously transmit packets within the TXOP period.